1. Field of the Invention
This invention concerns compositions suitable for use fluxes for brazing or welding in the aluminium fabrication industries.
2. Brief Description of Related Art
It has been common practice to use a flux to assist in the brazing or welding of aluminium in the radiator or other aluminium fabrication industries. The flux acts to destroy the oxide film which forms on aluminium surfaces thereby improving the flow of molten aluminium and/or of the molten filler or other jointing material.
Fluxes based on chlorides or on mixtures of chlorides with fluorides are known but have disadvantages because they give rise to corrosion in the aluminium and require to be removed from the brazed or welded article by water washing.
As an improvement over chloride fluxes, fluxes based on alkali metal, particularly potassium fluoaluminates, have been developed. Such fluxes may be prepared by mixing individual presynthesised alkali metal fluoaluminates, alkali metal fluorides or aluminium fluorides to desired proportions.
A substantial proportion of fluoaluminate fluxes are produced by wet chemical processes. A flux described in U.S. Pat. No. 4579605 of Furukuwa Aluminium Co. comprises 5% to 95 wt % of K.sub.2 AlF.sub.5 or K.sub.2 AlF.sub.5.H.sub.2) and the remainder of KAlF4 and may be prepared by dissolving aluminium hydroxide in hydrofluoric acid having a concentration of 5% to 40% by weight to give an Al:F ratio of 1:4 to 1:4.5 and neutralising this mixture with potassium carbonate or hydroxide to give a Al:K ratio of 1:1-1.5.
Japanese Patent Specification No. 60-87993 of Sumitomo Light Metal Industries describes a fluoaluminate type flux which has the advantage of a slightly lower working temperature. This flux consists essentially of a mixture of b 60% to 70% wt of K.sub.3 AlF.sub.6, 27% to 37% wt of AlF.sub.3 and 0.1% to 10% wt of KBF.sub.4 in which the KBF.sub.4 component and its proportion are critical. It may be prepared by mixing the three above identified compounds which mixture may be used as such or after fusing, solidifying and pulverising the solidified material into a powder.